Lightweight, high refractive index glasses for spectacle lenses

ABSTRACT

A glass having a high refractive index, preferably &gt;1.77, and a low specific gravity, preferable ≦3.8, is provided for utilization as spectacle lenses. The glass consists essentially of 80 mol percent or more of light cationic oxides with the remainder being cationic oxides having a greater molecular weight. It has a high crystallization stability in a viscosity range &gt;200 poises and an expansion coefficient α×10 7  between 20°-300° C. of ≦100. Sufficiently light in weight to be well-suited for use as spectacle lenses, the glass can be continuously produced, as it has no pronounced tendency toward crystallization. The resulting corrective optical lenses have relatively high dioptric values, even with small center and edge thicknesses.

BACKGROUND OF THE INVENTION

This invention relates to glasses having a high refractive index and a low specific gravity for use as spectacle lenses.

In order to replace the hitherto lightest-weight and highest refractive index glasses (nd=1.70, s=3.0) used as spectacle lenses by an even higher refractive index glass (e.g., nd>1.79), so as to produce spectacle lenses having higher dioptric values with even smaller edge thicknesses (for negative lenses) and/or even smaller center thicknesses (for positive lenses) than previously done, thereby enabling further enlargement of the optically effective field of vision while also improving cosmetic appearances, the specific gravity must be approximately 3.3 if the weight of the glass lens is to remain approximately the same. In a glass with a refractive index of nd=1.79 and a specific gravity of approximately 3.7, the eventual glass weight increase is approximately 12 percent for negative and positive lenses as compared to a glass having a refractive index of nd=1.70 and a specific gravity of 3.0.

Optical glasses with refractive indexes nd of 1.76-1.80 and specific gravities s of ≦4.5 are known, but as a result of their weight are not suited for use as glasses for spectacle lenses, even though such glasses can be produced by continuous mass-production means.

Examples of these unsuitable glasses are heavy flint glasses as follows:

    ______________________________________                                         SF 6    nd = 1.8051  vd = 25.43  s = 5.18                                      SF 11   nd = 1.7847  vd = 25.76  s = 4.74                                      SF 14   nd = 1.7612  vd = 26.53  s = 4.54                                      ______________________________________                                    

The known LaF glasses with a refractive index nd of >1.77 but a lower dispersion than heavy flint glasses have specific gravities of >4.0. Examples of the known LaF and LaSF glasses are as follows:

    ______________________________________                                         LaF 25    nd = 1.7843                                                                               vd = 41.30                                                                               s = 4.45                                        LaF 22    nd = 1.7818                                                                               vd = 37.1 s = 4.21                                        Laf 9     nd = 1.7950                                                                               vd = 28.39                                                                               s = 4.96                                        LaSF N 3  nd = 1.8080                                                                               vd = 40.75                                                                               s = 4.68                                        LaSF 8    nd = 1.80741                                                                              vd = 31.61                                                                               s = 4.87                                        ______________________________________                                    

Further high refractive index optical glasses, e.g., as disclosed by German Patent Specification No. 12 60 712, having refractive indexes nd of 1.75-1.80 and specific gravities ≦4.0 are known, but they are not suitable for continuous production of solid glasses (e.g., glasses for spectacle lenses) and crucible melting due to their pronounced tendency towards crystallization. This crystallization tendency is determined by the magnitude of the crystal growth speed and the crystallization temperature range in relation to viscosity.

Glasses having a refractive index >1.75 consist of 14-46 percent by weight of alkaline earth metal oxides, including a heavy BaO content ≦10 percent by weight. The ZnO content is 10-20 percent by weight and the TiO₂ -content is 10-25 percent by weight. Such glasses contain 12-20 percent by weight of SiO₂ and 8-20 percent by weight of B₂ O₃ as glass components, the sum of glass forming components being <31 percent by weight. The color of such high titanium-bearing glasses must be influenced by rapid cooling in such a manner that the glass has a strongly marked titanium color (yellow-brown). The glasses must be cast at a viscosity <100 poises because their strong tendency towards crystallization precludes tank production and automatic processing at higher viscosities, e.g. between 200 to 600 poises. The ZnO content of 10-20 percent by weight in conjunction with the high TiO₂ -content of 10-25 percent by weight is unfavorable for these glasses.

OBJECTS OF THE INVENTION

Accordingly, it is a general object of the present invention to provide a glass having a high refractive index and a low specific gravity which can be utilized in spectacle lenses.

Another object of the present invention is to provide a glass which has high crystallization stability within a given viscosity range in order that it may be continuously produced, good chemical resistance and a thermal expansion coefficient α×10⁷ between 20°-300° C.≦100.

An additional object of the present invention is to provide a glass which can be used to produce spectacle lenses having high dioptric values with smaller edge or center thicknesses than lenses which are currently known.

A more particular object of the present invention is to form a series of new glasses to be used in spectacle manufacture which have refractive indexes nd of 1.77-1.81 and specific gravities s of ≦3.8 (particularly those within the 3.4-3.8 range), additionally having the following characteristics in order to be technically viable from a manufacturing and practical viewpoint:

(a) high crystallization stability in a viscosity range of >200 poises to allow continuous production;

(b) a thermal expansion coefficient α×10⁷ between 20°-300° C. of ≦100; and

(c) good chemical resistance.

Upon study of the specification and appended claims, further objects, features and advantages of the present invention will become more fully apparent to those skilled in the art to which this invention pertains.

SUMMARY OF THE INVENTION

Briefly, the above and other objects, features and advantages of the present invention are attained in one aspect thereof by providing a glass for spectacle lenses having a refractive index nd of 1.77-1.81 and a specific gravity s of 3.4-3.8 which consists essentially of:

(a) more than 80 mol percent of mono and polyvalent cationic oxides having a molecular weight ≦100, namely

    ______________________________________                                         SiO.sub.2    22-30 mol percent                                                 B.sub.2 O.sub.3                                                                             17-23 mol percent                                                 SiO.sub.2 + B.sub.2 O.sub.3                                                                 40-49 mol percent                                                 Alkali metal oxides                                                                         0- 8 mol percent                                                  CaO          12-20 mol percent                                                 MgO + CaO    12-22 mol percent                                                 TiO.sub.2    17-24 mol percent; and correspondingly                            ______________________________________                                    

(b) less than 20 mol percent of cationic oxides having a molecular weight >100, namely

    ______________________________________                                         La.sub.2 O.sub.3                                                                              4-8 mol percent                                                 P.sub.2 O.sub.5                                                                               0-1 mol percent                                                 BaO            0-5 mol percent                                                 SrO            3-10 mol percent                                                BaO + SrO      5-10 mol percent                                                PbO            0-2 mol percent                                                 Al.sub.2 O.sub.3                                                                              0-1 mol percent                                                 ZrO.sub.2      0-2 mol percent                                                 Nb.sub.2 O.sub.5                                                                              0-1.5 mol percent                                               Ta.sub.2 O.sub.5                                                                              0-1.5 mol percent                                                                               0-2                                            WO.sub.3       0-0.5 mol percent                                               ______________________________________                                    

the glass having no pronounced tendency towards crystallization as determined by the magnitude of the crystal growth speed and the crystallization temperature range in relation to viscosity.

DETAILED DESCRIPTION

The new glasses according to the invention consist mainly (>80 mol percent) of light cationic oxides (having molecular weights of <100), whereby polyvalent components such as La₂ O₃ (<8 mol percent) and eventually Y₂ O₃ cannot be excluded. Y₂ O₃ reduces the specific gravity in relation to La₂ O₃. La₂ O₃ may be replaced extensively by Y₂ O₃, but only in small proportions of Y₂ O₃ where this is combined with a reduction in crystallization tendencies as defined hereinabove.

The porportion of glass-forming components SiO₂ +B₂ O₃ (+P₂ O₅ +GeO₂) is 30-40 percent by weight, the SiO₂ content being greater than the B₂ O₃ content.

The alkali metal content of such glasses is restricted within the following limits:

    ______________________________________                                         Li.sub.2 O 0-5                                                                 Na.sub.2 O 0-5          0-5 percent by weight                                  K.sub.2 O  0- 5                                                                ______________________________________                                    

so that the required expansion coefficient α×10⁷ can be adjusted between 20°-300° C. to ≦100.

The sum of alkaline earth metal components should be 19-25 percent by weight, the BaO content being no higher than 8 percent by weight, the BaO+SrO content being <15 percent by weight, and the MgO+CaO content being 7-15 percent by weight.

The glasses formed in accordance with the present invention consequently consist essentially of:

(a) more than 80 mol percent of mono to polyvalent cationic oxides with molecular weights ≦100, namely

    ______________________________________                                         SiO.sub.2    22-30 mol percent                                                 B.sub.2 O.sub.3                                                                             17-23 mol percent                                                 SiO.sub.2 + B.sub.2 O.sub.3                                                                 40-49 mol percent                                                 Alkali metal oxides                                                                         0- 8 mol percent                                                  CaO          12-20 mol percent                                                 MgO + CaO    12-22 mol percent                                                 TiO.sub.2    17-24 mol percent; and correspondingly                            ______________________________________                                    

(b) less than 20 mol percent of cationic oxides having a molecular weight of >100, namely

    ______________________________________                                         La.sub.2 O.sub.3                                                                              4-8 mol percent                                                 P.sub.2 O.sub.5                                                                               0-1 mol percent                                                 BaO            0-5 mol percent                                                 SrO            3-10 mol percent                                                BaO + SrO      5-10 mol percent                                                PbO            0-2 mol percent                                                 Al.sub.2 O.sub.3                                                                              0-1 mol percent                                                 ZrO.sub.2      0-2 mol percent                                                 Nb.sub.2 O.sub.5                                                                              0-1.5 mol percent                                               Ta.sub.2 O.sub.5                                                                              0-1.5 mol percent                                                                               0-2                                            WO.sub.3       0-0.5 mol percent                                               ______________________________________                                    

The glasses made in accordance with the present invention consist (in percent by weight) essentially of:

    ______________________________________                                         SiO.sub.2      15-20 weight percent                                            B.sub.2 O.sub.3                                                                               10-20 weight percent                                            CaO            8-15 weight percent                                             SiO.sub.2 + B.sub.2 O.sub.3                                                                   30-40 weight percent                                            La.sub.2 O.sub.3                                                                              16-26 weight percent                                            TiO.sub.2      15-25 weight percent                                            MgO            0-3 weight percent -SrO                                                                       1.0-10 weight percent                                                                8-15                                       BaO            0-8 weight percent                                              ZnO            0-4 weight percent                                              PbO            0-2 weight percent                                              Alkaline earth metal                                                           oxides         20-25 weight percent                                            Al.sub.2 O.sub.3                                                                              0-2 weight percent                                              ZrO.sub.2      0-3 weight percent                                              Nb.sub.2 O.sub.5                                                                              0-5 weight percent                                              Ta.sub.2 O.sub.5                                                                              0-2 weight percent                                              Alkali metal oxides                                                                           0-5 weight percent                                              P.sub.2 O.sub.5                                                                               0-3 weight percent                                              GeO.sub.2      0-3 weight percent                                              ______________________________________                                    

The sum of divalent components should be 20-29 percent by weight, the CaO+MgO content not being more than 15 percent by weight, since the crystallization tendency will otherwise increase considerably.

ZnO may be added to adjust the expansion coefficient up to 4 percent by weight; it is only with higher ZnO contents that the crystallization and segregation tendency increases.

P₂ O₅ or GeO₂ can be used in smaller proportions, providing that a maximum crystallization stabilization is achieved.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following examples, therefore, are to be construed as merely illustrative and not limitative of the remainder of the disclosure in any way whatsoever. Unless otherwise indicated, all pressures and temperatures are ambient and all parts and percentages are by weight.

The tables give specific examples of glasses formed according to the composition range suggested by the invention. Table 1 shows the compositions of 12 typical glasses as a percentage by weight and Table 2 shows the same glasses listed in molar percentages.

                                      TABLE 1                                      __________________________________________________________________________     Composition in percent by weight                                               Oxide  1   2   3   4   5   6   7   8   9   10  11  12                          __________________________________________________________________________     SiO.sub.2                                                                             15.0                                                                               17.4                                                                               16.3                                                                               17.0                                                                               18.6                                                                               18.8                                                                               19.4                                                                               18.6                                                                               18.0                                                                               19.7                                                                               19.5                                                                               18.7                        B.sub.2 O.sub.3                                                                       16.0                                                                               17.4                                                                               17.3                                                                               17.0                                                                               15.6                                                                               15.8                                                                               15.6                                                                               15.6                                                                               15.2                                                                               16.0                                                                               15.6                                                                               15.7                        SiO.sub.2 + B.sub.2 O.sub.3                                                           31.0                                                                               34.8                                                                               33.6                                                                               34.0                                                                               34.2                                                                               34.6                                                                               35.0                                                                               34.2                                                                               33.2                                                                               35.7                                                                               35.1                                                                               34.4                        P.sub.2 O.sub.5                                                                       --  --  --  1.0 --  --  --  --  --  --  --  --                          Li.sub.2 O                                                                            --  --  --  --  --  0.8 --  --  2.4 --  --  --                          Na.sub.2 O                                                                            --  --  --  --  --  0.3 --  --  0.9 0.1 --  2.0                         K.sub.2 O                                                                             --  --  --  --      0.2 --  --  0.5 --  --  --                          Alkali metal                                                                   oxides --  --  --  --  --  1.3 --  --  3.8 0.3 --  2.0                         MgO    --  --  --  --  1.9 1.0 1.0 --  --  --  1.0 --                          CaO    11.5                                                                               11.8                                                                               11.7                                                                               12.4                                                                               8.8 12.4                                                                               8.8 8.8 8.5 8.5 8.8 9.9                         MgO + CaO                                                                             11.5                                                                               11.8                                                                               11.7                                                                               12.4                                                                               10.7                                                                               13.4                                                                               9.8 8.8 8.5 8.5 9.8 9.9                         BaO    8.0 3.0 3.1 1.0 2.0 --  2.0 2.0 2.8 --  2.0 2.0                         SrO    5.0 8.2 8.1 8.0 9.8 7.9 9.8 9.8 9.5 8.0 9.8 8.0                         BaO + SrO                                                                             13.0                                                                               11.2                                                                               11.2                                                                               9.0 11.8                                                                               7.9 11.8                                                                               11.8                                                                               12.3                                                                               9.0 11.8                                                                               10.0                        ZnO    --  --  --  --  --  --  --  --  --  4.0 --  --                          PbO    --  --  --  --  --  --  2.0 --  --  --  --  --                          Al.sub.2 O.sub.3                                                                      1.0 --  --  --  0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2                         La.sub.2 O.sub.3                                                                      26.3                                                                               24.6                                                                               26.0                                                                               22.4                                                                               23.4                                                                               19.1                                                                               23.4                                                                               23.4                                                                               16.1                                                                               19.5                                                                               22.8                                                                               23.2                        Y.sub.2 O.sub. 3                                                                      --  --  --  1.0 --  1.0 --  --  --  1.0 --  --                          Bi.sub.2 O.sub.3                                                                      --  --  --  --  --  --  --  --  --  --  --  --                          ZrO.sub.2                                                                             --  --  --  --  --  2.0 --  --  --  2.0 --  --                          TiO.sub.2                                                                             17.0                                                                               17.4                                                                               17.3                                                                               17.0                                                                               16.6                                                                               16.8                                                                               16.6                                                                               16.5                                                                               22.9                                                                               17.1                                                                               16.6                                                                               17.8                        Ta.sub.2 O.sub.5                                                                      --  --  --  --  --  --  --  --  --  --  0.5 1.0                         Nb.sub.2 O.sub.5                                                                      --  --  --  3.0 2.9 3.0 3.0 2.9 2.8 3.0 3.0 1.5                         WO.sub.3                                                                              --  --  --  --  --  0.5 --  --  --  0.5 --  --                          As.sub.2 O.sub.3                                                                      0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 --                          NaCl   --  --  --  --  --  --  --  --  --  0.2 --  --                          nd     1.7992                                                                             1.7892                                                                             1.7958                                                                             1.7928                                                                             1.7927                                                                             1.7846                                                                             1.7893                                                                             1.7986                                                                             1.8115                                                                             1.7737                                                                             1.7889                                                                             1.7861                      vd     34.1                                                                               34.4                                                                               34.3                                                                               33.7                                                                               33.6                                                                               33.7                                                                               33.8                                                                               33.1                                                                               30.4                                                                               33.2                                                                               33.6                                                                               33.4                        s      3.78                                                                               3.67                                                                               3.71                                                                               3.60                                                                               3.67                                                                               3.54                                                                               3.66                                                                               3.74                                                                               3.54                                                                               3.56                                                                               3.66                            __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     Composition in mol percent                                                     Oxide  1   2   3   4   5   6   7   8   9   10  11  12                          __________________________________________________________________________     SiO.sub.2                                                                             22.9                                                                               25.3                                                                               24.1                                                                               24.9                                                                               27.2                                                                               26.0                                                                               28.6                                                                               28.1                                                                               24.4                                                                               28.5                                                                               28.7                                                                               27.3                        B.sub.2 O.sub.3                                                                       21.2                                                                               21.9                                                                               22.1                                                                               21.5                                                                               19.7                                                                               18.9                                                                               19.8                                                                               20.4                                                                               17.8                                                                               19.9                                                                               19.8                                                                               19.8                        SiO.sub.2 + B.sub.2 O.sub.3                                                           44.1                                                                               47.2                                                                               46.2                                                                               46.4                                                                               46.9                                                                               44.9                                                                               48.4                                                                               48.5                                                                               42.2                                                                               48.4                                                                               48.5                                                                               47.1                        P.sub.2 O.sub.5                                                                       --  --  --   0.6                                                                               --  --  --  --  --  --  --  --                          Li.sub.2 O                                                                            --  --  --  --  --  2.2 --  --  6.5 --  --  --                          Na.sub.2 O                                                                            --  --  --  --  --  0.4 --  --  1.2 0.4 --  2.8                         K.sub.2 O                                                                             --  --  --  --      0.2 --  --  0.4 --  --  --                          Alkali metal                                                                   oxides --  --  --  --  --  2.8 --  --  8.1 0.4 --  2.8                         MgO    --  --  --  --  4.1 2.1 2.2 --  --  --  2.2 --                          CaO    18.8                                                                               18.4                                                                               18.6                                                                               19.4                                                                               13.8                                                                               18.4                                                                               13.9                                                                               14.3                                                                               12.3                                                                               13.2                                                                               13.9                                                                               15.5                        MgO + CaO                                                                             18.8                                                                               18.4                                                                               18.6                                                                               19.4                                                                               17.9                                                                               20.5                                                                               16.1                                                                               14.3                                                                               12.3                                                                               13.2                                                                               16.1                                                                               15.5                        BaO    4.8  1.7                                                                                1.8                                                                                0.6                                                                               1.1 --  1.2 1.2 1.5 --  1.1 1.1                         SrO    4.4  6.9                                                                                6.9                                                                                6.8                                                                               8.3 6.3 8.4 8.6 7.4 6.7 8.4 6.8                         BaO + SrO                                                                             9.2  8.6                                                                                8.7                                                                                7.4                                                                               9.4 6.3 9.6 9.8 8.9 6.7 9.5 7.9                         ZnO    --  --  --  --  --  --  --  --  --  4.3 --  --                          PbO    --  --  --  --  --  --  --  0.8 --  --  --  --                          Al.sub.2 O.sub.3                                                                      0.9 --  --  --  0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2                         La.sub.2 O.sub.3                                                                      7.4  6.6                                                                                7.1                                                                                6.0                                                                               6.3 4.9 6.3 6.5 4.0 5.2 6.7 6.2                         Y.sub.2 O.sub.3                                                                       --  --  --   0.4                                                                               --  --  --  0.4 --  --                                  Bi.sub.2 O.sub.3                                                                      --  --  --  --  --  --  --  --  --  --  --  --                          ZrO.sub.2                                                                             --  --  --  --  --  1.3 --  --  --  1.4 --  --                          TiO.sub.2                                                                             19.5                                                                               19.1                                                                               19.3                                                                               18.7                                                                               18.2                                                                               17.5                                                                               18.4                                                                               18.8                                                                               23.3                                                                               18.6                                                                               18.4                                                                               19.6                        Ta.sub.2 O.sub.5                                                                      --  --  --  --  --  --  --  --  --  --  0.1 0.2                         Nb.sub.2 O.sub.5                                                                      --  --  --   1.0                                                                               1.0 0.9 0.9 1.0 0.9 0.9 0.9 0.5                         WO.sub.3                                                                              --  --  --  --  --  0.2 --  --  --  0.2 --  --                          As.sub.2 O.sub.3                                                                      0.1  0.1                                                                                0.1                                                                                0.1                                                                               0.1 0.1                                                                            0.1 0.1 0.1 0.1 0.1 --                              NaCl   --  --  --  --  --  --  --  --  --  --  --  --                          __________________________________________________________________________

Melting Example

Composition:

    ______________________________________                                                                     Kg amounts weighed in                                     Percent    Starting  for a 90 kg computed                               Oxides by Weight  Materials quantity of glass                                  ______________________________________                                         SiO.sub.2                                                                             18.7       SiO.sub.2 16.926                                             B.sub.2 O.sub.3                                                                       15.7       H.sub.3 BO.sub.3                                                                         25.296                                             Li.sub.2 0                                                                            0.8        Li.sub.2 CO.sub.3                                                                        1.769                                              Na.sub.2 O                                                                            0.3        Na.sub.2 CO.sub.3                                                                        0.455                                              K.sub.2 O                                                                             0.2        K.sub.2 CO.sub.3                                                                         0.267                                              MgO    1.0        MgCO.sub.3                                                                               2.292                                              CaO    12.4       CaCO.sub.3                                                                               20.059                                             SrO    7.9        Sr(NO.sub.3).sub.2                                                                       14.457                                             Al.sub.2 O.sub.3                                                                      0.2        A10(OH)   0.237                                              La.sub.2 O.sub.3                                                                      19.1       La.sub.2 O.sub.3                                                                         17.184                                             Y.sub.2 O.sub.3                                                                       1.0        Y.sub.2 O.sub.3                                                                          0.889                                              TiO.sub.2                                                                             16.8       TiO.sub.2 15.168                                             ZrO.sub.2                                                                             2.0        ZrO.sub.2 1.798                                              Nb.sub.2 O.sub.5                                                                      3.0        Nb.sub.2 O.sub.5                                                                         2.668                                              NaCl   0.2        NaCl      0.178                                              WO.sub.3                                                                              0.5        WO.sub.3  0.445                                              As.sub.2 O.sub.3                                                                      0.2        As.sub.2 O.sub.3                                                                         0.178                                              ______________________________________                                    

After weighing in, the starting materials are mixed in a commercial mechanical mixer. The mixture is melted down, for example, in a Pt-crucible at 1200°-1220° C., refined at 1280° C. for 4 hours and then stirred until the temperature drops to about 1060° C. The resultant melt is drained through a Pt-pipe and is processed into a bar or a blob.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art to which this invention pertains can easily ascertain the essential characteristics thereof and, without departing from the spirit and scope of the present invention, can make various changes and modifications to adapt it to various usages and conditions. 

What is claimed is:
 1. A glass for spectacle lenses having a refractive index nd of 1.77-1.81, a dispersion nd of 30.4-34.4 a specific gravity s of 3.4-3.8 and a thermal expansion coefficient α×10⁷ at 20°-200° C. of ≦100, said glass consisting essentially of:(a) more than 80 mol percent of mono and polyvalent cationic oxides having a molecular weight ≦100, namely

    ______________________________________                                         SiO.sub.2    22-30 mol percent                                                 B.sub.2 O.sub.3                                                                             17-23 mol percent                                                 SiO.sub.2 + B.sub.2 O.sub.3                                                                 40-49 mol percent                                                 Alkali metal oxides                                                                         0- 8 mol percent                                                  CaO          12-20 mol percent                                                 MgO + CaO    12-22 mol percent                                                 TiO.sub.2    17-24 mol percent; and correspondingly                            ______________________________________                                    

(b) less than 20 mol percent of cationic oxides having a molecular weight of >100, namely

    ______________________________________                                         La.sub.2 O.sub.3                                                                              4-8 mol percent                                                 P.sub.2 O.sub.5                                                                               0-1 mol percent                                                 BaO            0-5 mol percent                                                 SrO            3-10 mol percent                                                BaO + SrO      5-10 mol percent                                                PbO            0-2 mol percent                                                 Al.sub.2 O.sub.3                                                                              0-1 mol percent                                                 ZrO.sub.2      0-2 mol percent                                                 Nb.sub.2 O.sub.5                                                                              0-1.5 mol percent                                               Ta.sub.2 O.sub.5                                                                              0-1.5 mol percent                                                                               0-2                                            WO.sub.3       0-0.5 mol percent                                               ______________________________________                                    

said glass further having no pronounced tendency toward crystallization in a viscosity range of >200 poises as determined by the magnitude of the crystal growth speed and the crystallization temperature range in relation to the viscosity.
 2. A glass composition according to claim 1, having a refractive index nd of 1.7739-1.7992 and a specific gravity s of 3.54-3.78, comprising:

    ______________________________________                                         SiO.sub.2        22.9-28.7 mol percent                                         B.sub.2 O.sub.3  17.8-22.1 mol percent                                         SiO.sub.2 + B.sub.2 O.sub.3                                                                     42.2-48.5 mol percent                                         MgO + CaO        12.3-21.5 mol percent                                         TiO.sub.2        17.5-23.3 mol percent;                                                         and correspondingly                                           ______________________________________                                    

(b) less than 20 mol percent of cationic oxides having a molecular weight of >100, namely

    ______________________________________                                         La.sub.2 O.sub.3 4.0-7.4 mol percent                                           P.sub.2 O.sub.5  0-1 mol percent; and                                          BaO + SrO        6.3-9.8 mol percent.                                          ______________________________________                                    


3. A glass composition according to claim 2, containing 12.3-19.4 mol percent CaO.
 4. A glass composition according to claim 3, containing 4.4-8.6 mol percent SrO. 